Chap. VII.] 



BONE. 



61 



74. IiUermembranous formation. All 



bones not preformed in the embryo as cartilage are 

 developed directly from the periosteum in the manner 

 of the periosteal bone just described (Fig. 37). Here also 



Fig. 37. A small mass of Bone Substance in the Periosteum of the 

 Lower Jaw of a Human Foetus. 



a, Osteogenetic layer of periosteum ; 6, multi-nucleated giant cells, myeloplaxes. 

 The one in the middle of the upper margin is an osteoclast, whereas the 

 smaller one at the left upper corner appears concerned in the formation of 

 bone. Above c the osteoblast cells become surrounded by osseous substance 

 and thus become converted into bone-cells. (Atlas.) 



the new bone is at first spongy bone, which in its inner 

 layers gradually becomes converted into compact bone. 



In all instances during embryonic life and after 

 birth the growth of a bone in thickness takes place 

 after the manner of periosteal bone ; this is at first 

 spongy, but is gradually converted into compact bone. 



75. All osseous substance is formed in the embryo 

 and after birth by the osteoblasts, or marrow cells 

 (Gegenbaur, Waldeyer) : each osteoblast giving origin 

 to a zone of osseous matrix, and remaining in the 



